管材的直徑根據所屬管材的材質不同，其意義也不一樣，金屬管道如鋼管，鐵管等所指的管直徑是指內徑，而PE管，PPR管等都是指外徑，標注上DN指內徑，DE指外徑，我們常說的4分管，6分管，1寸管是英制的標準，4分管是 G1/2英寸的俗稱，6分管也就是G3/4的意思

   公稱通徑DN/mm        俗     稱              相當管螺紋/in           外徑 DE/mm

               6                        1分管                       1/8                                 10

               8                        2分管                       1/4                                 

              10                      3分管                       3/8                             14或者17

              15                      4分管                       1/2                             18或者22

              20                      6分管                        3/4                             25或者27

              25                      1寸管                         1                                32或者34